Recently, roller screws have been developed, in each of which rollers are arranged between a screw shaft and a nut so that the rollers can be rolled. Because of a linear contact between the rollers and a roller-rolling surface, the roller screws are advantageous over ball screws in that the roller screws are able to sustain an amount of load larger than that of the ball screws. However, when being compared to balls which are able to roll in all directions, the roller screws are disadvantageous in that it is very difficult to circulate the rollers due to the fact that the rollers are allowed to roll along only one direction.
The roller screw has a circulation path configured by a loaded roller-rolling path, which is produced between a spiral roller-rolling surface of the outer peripheral surface of the screw shaft and a spiral loaded roller-rolling surface of an inner peripheral surface of the nut, and an unloaded return path connecting one end and the other end of the loaded roller-rolling path. The unloaded return path is formed on a circulation member attached to the nut. When the screw shaft is rotated relatively to the nut, the circulation member scoops up the rollers that have rolled and arrived at the one end of the loaded roller-rolling path, and allows the rollers to pass through the unloaded return path, so that the rollers returns to the other end of the loaded roller-rolling path. Arranging the circulation member makes it possible to form the unloaded return path.
In the conventional roller screw, the number of turns of the loaded roller-rolling path is set to be larger than 2 or 3. When a return pipe is used as the circulation member, the number of turns of the loaded roller-rolling path is set to, for example, 2.5 turns, 3.5 turns, or the like (refer to Patent reference 1). Meanwhile an end-cap type of circulation member is adopted, the number of turns of the loaded roller-rolling path is set to, for example, 3.8 turns, 4.8 turns, or the like (refer to Patent reference 2).    Patent reference 1: Japanese Patent Laid-open Publication No. 2001-241527    Patent reference 2: Japanese Patent Laid-open Publication No. 2006-118649